Fic where Harry is the master of death and travels the multiverse helping out other main characters and getting a large harem of lovers to make his dream of a large family come true?
I believe this may be "A MODern Man" by dogbertcarroll
"If you're really curious on how things could have turned out differently you could take the place of one of your dead alternates," Luna suggested.
"Pardon?" Harry asked.
"You're the Master of Death," Luna pointed out. "That means more than just a nifty title. You can take the place of one of the infinite alternate versions of you that have passed on."
"You mean like alternate realities?"
"Exactly," Luna agreed. "Simply concentrate on your connection to the Hallows and feel for the life you want and then decide to make the switch."
Harry wondered if it could truly be that easy as he closed his eyes and imagined it his form fading into a ghostly outline before flickering out.
Lune stared wide eyed. "But… I was just fucking with you!"
I've read a few of dogbertcarroll's other stories, so the "Mod!Harry" bit struck me.